"I miss my older brother. I want to see him right now."
Both are acceptable.
This sentence can actually be said in 3 ways, with no difference in meaning#:
If I am the person to say this, it probably would depend on how close the listener is to me and to my brother. If he knows both of us well (e.g. a 3rd person in my family), it will be A; If he knows me well but not my brother (e.g. has only heard I have a brother), it will be B; If he doesn't know us well, or doesn't even know I have a brother, it will be C. After all, it is all about how much information the listener requires and where we want the emphasis to be.
(# - for the convenience of discussion only.)
They are similar but 现在 is neutral and purely a time reference (now). 马上 would rather mean "immediately", "in no time".
大家早上好！现在是上午七点。（马上XX）/ Good morning everybody! It's 7 o'clock.
情况很差，请马上去看看！（现在 OK）/ The situation is bad, please go there and check it immediately!